bibtex.py: Update date format in generated bibtex

In https://gitlab.softwareheritage.org/swh/devel/swh-indexer/-/blob/master/swh/indexer/bibtex.py?ref_type=heads (l. 109).

Example of generated BibTeX:

@softwareversion{REPLACEME,
    author = "Di Cosmo, Roberto and Danelutto, Marco",
    organization = "Inria and University Paris Diderot and University of Pisa",
    license = "LGPL-2.0-only",
    date = "2011-07-18",
    year = "2011",
    month = "07",
    repository = "git+https://github.com/rdicosmo/parmap.git",
    title = "Parmap",
    version = "1.2.5"
}

Roberto: [...] normalement si on a un champ "date" il n'est pas nécessaire de ajouter aussi "year" et "month" parce-que c'est redondant, mais si on les ajoute, pour compatibilité avec les vieux styles il convient de utiliser pour "month" dans BibTeX les littérales: jan, feb, mar, apr, may, jun, jul, aug, sep, oct, nov, dec (ce sont des macros, donc pas de " ou {}, par ex: month=jan,).

However, the current library (pybtex) does not allow generating macros.

• Should we switch lib?
• Should we leave it as is?
• Is it better if we use another format for month (https://www.bibtex.com/f/month-field/) which is not a macro? Let's say, a number (e.g. month = 2).

To discuss with @douardda, @vlorentz and Roberto.